This invention relates to the use of oxygenation compounds, and mixtures of these oxygenation compounds with phosphates or surfactants, or both, as agents which foster the growth of soil microorganisms that digest pollutants in the soil. In particular, this invention relates to the use of certain metal peroxides or metal peroxide/phosphate/surfactant mixtures (oxygen releasing compounds, or "ORCs") by directly applying the ORC to the soil or blending the ORC with plant nutrients, or other beneficial additives, or both, and then applying to the soil.
Wildlife, water quality and human safety are all threatened by the presence of certain waste chemicals in soil and water supplies. These chemicals are capable of persisting in the environment undisturbed for long periods of time and can be classified as environmental pollutants. Investigators have looked to the degradative capacity of microorganisms in order to harness the ability of some bacteria, fungi and protozoa to break down waste compounds. Typically, such waste compounds are organic chemicals such as hydrocarbons or halocarbons. However, the definition also extends to inorganics including certain toxic ions such as heavy metals and radioisotopes.
Bioremediation refers broadly to the use of microbiological populations to participate in the biodegradation, transformation or sequestration of a given environmental pollutant. In situ biodegradation by microorganisms has been documented in field studies of ponds and soil (J.C. Spain, et al. Appl. Environ. Microbiol., 48:944. 1984), in which bacteria are used to break down organic compounds into carbon dioxide and water. Other soil decontamination procedures include soil washing and thermal treatment. These techniques are only partially satisfactory as some merely relocate the contaminant to an alternative site and others convert the pollutant to another undesirable form. In bioremediation, the organisms use the materials as a food source and convert them into useful or innocuous metabolites. Sometimes they sequester materials, e.g., heavy metals, that can actually be recovered for economic benefit.
Organisms that are native or foreign to a particular contaminated site can be employed in the bioremediation process; however, each individual contaminated location has soil compositions that are unique to that site. Populations of organisms evolve based on the selective pressures they receive from their surroundings. Thus, organisms native to a given location may be better adapted to survival in that location, or may have the genetic ability to metabolize an existing pollutant, and may therefore be better candidates to assist in biodegradation.
To date, aerobes, those organisms requiring oxygen for growth, are more frequently used for biodegradation than anaerobes. For some pollutants, however, bioremediation may be accomplished by anaerobes or sequential anaerobic-aerobic use cycles. Since an important aspect of bioremediation is to provide nutritional and environmental support to promote the growth of the appropriate bacteria and other organisms that can degrade the contaminant, oxygen, inorganic nutrients and other beneficial additives are added to the soil, through a variety of means, to increase the activity of the microbe population in an aerobic process.
Current technology often includes the excavation and relocation of contaminated soil (termed off site bioremediation) or excavation and treatment without relocation. The excavated soil is periodically turned over to ensure good aeration, if permitted by applicable air quality regulations, and the soil may be periodically moistened with water and supplemented with nutrients and other additives that promote bacterial growth. Air distribution systems can alternately be plumbed into the ground to oxygenate the soil; however, this can be impractical in high density media and may also be regulated by air quality standards.
Bioreactors have also been employed for biodegradation. In one form of bioreactor, soil is placed into a containment vessel which is rotated to maintain loose, aerated soil. This process has the disadvantage that it can be slow and expensive. Temperature, oxygen and nutrients are all controlled as needed. Off site techniques promote biodegradation but can be costly and time consuming. Soil must be transported to a site where it undergoes treatment for up to several years or more. While soil removal may be a necessity for gross contamination, some sites are too large to relocate.
There are several proposed methods for on site biodegradation. These often involve infiltrating the soil. Sometimes wells are dug and ground water is pumped to the surface. The water is purified, phosphates, nitrates and other nutrients are added, and the water is pumped through the soil.
U.S. Pat. No. 3,796,637 to Fusey states that the use of compositions of 10 to 40% by weight of iron oxide, manganese dioxide, zinc oxide or an alkali metal peroxide (monovalent series, e.g., sodium peroxide or potassium peroxide from group IA of the Periodic Table), promotes the biological degradation of hydrocarbon-containing waste material. The substances are said to promote biological degradation and to reduce the odors associated with anaerobic fermentation. While these compounds are stated in Fusey's examples to be useful for liquid-based biodegradation, it is not clear if they could be practical in promoting biodegradation in soil.
The addition of elemental oxygen, hydrogen peroxide, nitrate and surfactant are currently being tested to determine whether the addition of various combinations of these ingredients promote hydrocarbon degradation in the soil (Fouhy, K., et al. Chem. Engineer. March, 1991, pp. 30-35). L. Freidrich of Triachler (Darmstadt, Germany) indicated that hydrogen peroxide seems to be the most effective. Neither is admitted to be prior art by citation herein. The use of nitrates is disadvantageous because nitrate is a pollutant, and is not as efficient in delivering oxygen as the compounds disclosed herein.
There are a number of problems associated with the use of hydrogen peroxide in the soil either alone or in combination with fertilizers. Hydrogen peroxide is relatively unstable. In particular, formulations of hydrogen peroxide in combination with some metals can result in spontaneous combustion with increased temperatures. For example, the presence of Fe.sup.+2, a common ingredient in fertilizer mixtures, whether by design or trace contamination, can result in rapid destabilization of hydrogen peroxide.
Further, the average lifetime of hydrogen peroxide in the soil can be as little as several hours, depending on the soil conditions and the catalytic properties of its constituents. Thus, H.sub.2 O.sub.2 may not even survive long enough to make it to the desired treatment site. Hydrogen peroxide decomposition also results in the production of oxygen free radicals that are toxic to those same microorganisms whose growth is required for bioremediation.
Thus, repeated applications of relatively low hydrogen peroxide concentrations are required to foster bacterial growth without undue toxicity. Since the time required for bioremediation is proportional to the rate of bacterial replication and enzymatic activity, hydrogen peroxide based soil oxygenation still results in a lengthy, expensive and potentially hazardous biodegradation process.
Notwithstanding the foregoing, there remains a need for a method of enhancing in situ, excavated on site, or off site bioremediation by, stimulating either native microorganisms or innoculae or both, which provides for safe and effective time-release delivery of oxygen and other nutrients or other additives to a sufficient depth in a soil media.